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Toilet Leak: How to Find the Cause and Get It Fixed

Open toilet cistern showing its internal mechanism

A toilet leak is either a slow, silent waste of clean water from the cistern into the pan, or a visible puddle around the base. Both need a plumber’s attention, and the first is the one most people miss. A simple tissue test tells you in a few hours which one you have. If you are in London, we can fix the fault at a fixed price quoted before any work starts, and our emergency call out service answers 24/7 for water that will not stop.

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Stop a Toilet Leak Before It Worsens
Evo Plumbers provides London plumbing repairs, with 24/7 emergency coverage for urgent water issues.

TL;DR

  • Wipe the back of the pan dry, lay a fresh sheet of toilet tissue across it and leave it for up to three hours or overnight. Wet or torn paper means a leaky loo.
  • Water on the floor around the base, or a cistern that overflows, is a job for a plumber. Close the isolation valve next to the toilet first.
  • A leaking toilet can waste around 400 litres a day, according to WaterSafe, and the cost shows up on your bill.
  • Thames Water says a toilet leak must be repaired within 4 weeks of finding it.
  • Book a repair early: slow leaks can fall outside home insurance cover.

Table of Contents

How to tell if your toilet is leaking

Some leaks are obvious: a pool of water on the bathroom floor, a cistern that drips, a hiss that never stops. The expensive ones are quiet. Modern cisterns often overflow into the bowl rather than out through an external overflow pipe, so a fault can run for months without a drip showing anywhere. Severn Trent says around 1 in 20 toilets in the UK leaks and that they are not always easy to spot.

The tissue test

This is the check water suppliers themselves recommend:

  1. Flush, then wait about half an hour.
  2. Dry the back of the pan with toilet tissue.
  3. Lay a new, dry sheet of tissue across the back of the pan.
  4. Leave it for up to three hours, or overnight, and do not use the toilet.
  5. If the paper is wet or torn, you have a leaky loo.

The method is set out by WaterSafe Register, and Thames Water gives the same steps. Thames Water adds that water trickling, rippling or flowing at the back of the bowl is a sign in itself.

What to look and listen for

  • A faint hiss or trickle when nobody has flushed.
  • The cistern refilling on its own every few minutes.
  • A visible film of water running down the back of the bowl.
  • Water around the base, a damp patch on the ceiling below, or a soft or stained floor.
  • A cistern that takes a long time to refill after flushing.

A tissue test that comes back dry does not rule out a base leak. If the floor around the toilet is damp after you have used it, that is a separate fault from the cistern-to-pan leak above.

What causes a toilet to leak

The cause depends on where the water is going. Here is how to read the symptoms.

What you see Likely cause Who should fix it
Water trickling into the bowl, level in the cistern dropping Faulty flush valve (the seal that holds water in the cistern) Plumber
Water running into the overflow, cistern keeps refilling Faulty fill valve or float Plumber
Cistern slow to fill, or not filling Fill valve, supply valve or blockage in the inlet Plumber
Water at the base of the pan Worn pan connector or seal, loose fixings, or a cracked pan Plumber
Drip from pipes at the cistern Loose or worn connections Plumber

Cistern leaking or running into the bowl

Thames Water says leaky loos are usually caused by a faulty flush valve or fill valve inside the tank. Its test for the flush valve is to mark the water level in the cistern and check it again 10 minutes later. If the level has dropped, water is slipping past the seal. A fill valve problem shows as water running into the overflow tube, or a tank that takes a long time to refill.

Cambridge Water makes a similar point from the other direction. If the water in the cistern stays level while the pan is still leaking, the flush unit is probably the cause. If the water is high, rising and spilling into the bowl, the float is the likelier culprit.

Cistern water clues and likely toilet fault

Why newer toilets leak more quietly

The Bathroom Manufacturers Association has acknowledged, as reported in The Guardian, that the design of many dual-flush toilets makes them more prone to leakage. United Utilities explains that push button cisterns tend to overflow into the bowl, so faults are harder to spot and are nearly always the main cause of internal leaks. In practice this means a toilet that looks fine can be running all night.

Toilet leaking from the base

Water at the foot of the pan has a different set of causes from a cistern fault: a failed seal where the pan meets the waste pipe, loose fixings, a leaking connector, or damage to the pan itself. Because the toilet may need to be lifted and refitted to reach the fault, this is plumbing work rather than a quick adjustment. It is also the leak most likely to damage flooring, so do not leave it. Our plumbing work service covers it.

Is a leaking toilet an emergency?

It depends on the water. A slow cistern-to-pan leak is not an emergency, but it does have a deadline. A steady flow, an overflowing cistern, or water spreading across the floor is urgent.

Stop the water first

The Association of Plumbing and Heating Contractors (APHC) advises that smaller leaks in sanitaryware can be controlled with the isolation valve next to the appliance. For a major leak or flood, turn off the water at the main stop tap, which is often in the kitchen. Every property has two stop taps: the main one inside, and the water company’s boundary stop tap.

A quarter turn of the slotted valve near the cistern, with a flat screwdriver, usually isolates the toilet. Then flush once to empty the cistern, put towels down and call a plumber. If you cannot find or close the valve, shut the main stop tap and call us.

The 4-week rule and your insurance

Thames Water states that you must arrange repair of a toilet leak within 4 weeks of finding it, and that if it is not repaired the water company may need to step in to protect the supply. Other suppliers have their own policies, so check your own supplier’s page.

Insurance is the other reason not to wait. Uswitch explains that damage from slow leaks over time is often excluded from escape of water cover, because most policies respond to sudden and unexpected leaks rather than long-term deterioration. A leak you have known about for weeks is the kind insurers can class as maintenance.

How much water a leaking toilet wastes

The numbers are larger than most people expect.

  • The UK Government’s Defra consultation summary puts the average waste from a leaking toilet at between 215 and 400 litres of drinking water a day, with around 5 to 8% of UK toilets leaking.
  • At 400 litres a day, a month’s leak is roughly 12,000 litres. That is simple multiplication, not a measured figure for your home.
  • WaterSafe says a toilet leaking clean water from cistern to pan can waste up to 400 litres a day and add around £300 a year to a water bill if left unfixed.
  • Thames Water breaks it down by severity: a small trickle can waste up to 200 litres a day and cost an extra £161.33 a year, rippling water 600 litres a day at £483.99 a year, and constant flow 8,000 litres a day at £6,453.20 a year.

The cost lands hardest on metered homes, but unmetered households pay too, in the water that is wasted. Defra’s summary says leaks from toilets play a key role in preventing water waste, and the UK is committed to cutting leakage by 50% by 2050.

Leaks are common. South West Water reported in 2026 that it found leaking toilets in around one in four homes it visited, 738 across 2,874 home visits.

How much water a leaking toilet wastes

Fixing a leaking toilet

The cause decides the repair. A flush valve, fill valve or float fault means replacing the part. A base leak means isolating the water, lifting the pan, replacing the failed seal or connector and refitting it. Cracked sanitaryware or leaking pipework means a larger repair.

Cambridge Water notes that professional help is almost certainly needed if the toilet is boxed in or the leak comes from cracked sanitaryware or pipework. Water suppliers also say a fill valve or float replacement is a more complicated job than a seal fix, and one for a plumber. Most leaks in a home combine two of these: a worn part and a limescale-affected seal, which is why replacing the part and testing again is the sound route.

What a good repair visit looks like

  1. The plumber confirms the cause, not just the symptom.
  2. The price is agreed before any work starts.
  3. The faulty part is replaced and the toilet is tested through several flushes.
  4. The water level and the pan are checked afterwards, so the fault is not still running overnight.

At Evo Plumbers, every job is quoted at a fixed price before work starts, and you pay on completion. That means no surprise costs if the fault turns out to be the fill valve rather than the flush valve.

When to call a plumber straight away

Book a plumber without waiting for the tissue test if:

  • Water is on the floor around the base after every flush.
  • The cistern is overflowing or the flow does not stop.
  • You cannot close the isolation valve or find the stop tap.
  • There is a damp or stained ceiling below the bathroom.
  • The toilet rocks, or the pan or cistern is cracked.
  • The floor feels soft or looks stained.
  • The tissue test shows a leak and you want it fixed rather than monitored.

If the water supply is stopped and the toilet is out of use, that is the point to call, not to wait for morning. Our engineers cover all London boroughs, around the clock.

Commercial and rental properties

For landlords, a leaking toilet is a repair that tenants report and that can turn into floor and ceiling damage in the flat below. For businesses, water that flows to the drain is also charged for as sewage: Quensus notes wastewater is billed at 90 to 95% of the water reading on most commercial meters, so a silent leak costs twice. Fixing it early is cheaper than any of the alternatives.

Our view: fix the leak, not just the symptom

We would rather see a leak fixed on the first visit than watched for weeks. A toilet that runs quietly is the one that people put off, and it is the one that quietly costs the most.

Our approach is built around three things: calls answered 24/7, a fixed price quoted before work starts, and Gas Safe registered engineers. For a toilet leak, that means you know what the repair costs before anyone picks up a tool, and you are not left with a bathroom out of use overnight.

If the leak is hidden, in a wall, under a floor or in the pipework feeding the toilet, our leak detection service is the next step, so the cause is found rather than guessed.

the author

Get your toilet leak fixed in London

If the tissue test came back wet, or there is water around the base, we can send an engineer. Our plumbing work covers toilet repairs, and the quote is fixed before we start. For water that is spreading or will not stop, use our emergency call out, which is answered 24/7 across London.

Evoplumbers

If you suspect the leak is somewhere you cannot see, read our guide to leak detection in London. If pipework is the problem, our guide to pipe replacement explains when repair stops making sense.

FAQ

Why is my toilet leaking from underneath?

Water at the base usually points to a failed seal or connector between the pan and the waste pipe, loose fixings, or damage to the pan. It can also be water running down from a leaking cistern connection. Close the isolation valve, dry the area and see whether the floor stays dry after flushing. If water returns, book a plumber, because the toilet may need to be lifted to reach the fault.

Why is my toilet cistern not filling or filling slowly?

Thames Water says a fill valve problem shows as a tank that takes a long time to refill, or water running into the overflow tube. A partly closed isolation valve can also slow the refill. If the cistern still fills slowly with the valve fully open, the fill valve needs attention from a plumber.

How much water can a leaking toilet waste in a month?

Defra’s consultation summary gives an average of 215 to 400 litres a day, which is roughly 6,500 to 12,000 litres a month by simple multiplication. Thames Water shows that a constant flow can be far higher, up to 8,000 litres a day.

Is a leaking toilet classed as an emergency?

A slow leak into the pan is not an emergency, but it should be repaired soon, and Thames Water says within 4 weeks of finding it. Water on the floor, an overflowing cistern or a flow you cannot stop is urgent. Isolate the toilet, or turn off the main stop tap, then call us.

Sources

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